For more than half a century the cathode ray tube (CRT) has been the principal device for electronically displaying visual information. Although CRTs have been endowed during that period with remarkable display characteristics in the areas of color, brightness, contrast and resolution, they have remained relatively bulky and power hungry. The advent of portable computers has created intense demand for displays which are lightweight, compact, and power efficient. Liquid crystal displays (LCDs) are now used almost universally for lap-top computers. However, contrast is poor in comparison to CRTs, only a limited range of viewing angles is possible, and battery life is still measured in hours rather than days.
As a result of the drawbacks of LCD and CRT technology, field emission display (FED) technology has been receiving increased attention by industry. Flat panel displays utilizing FED technology employ a matrix-addressable array of cold, pointed field emission cathodes in combination with a luminescent phosphor screen. Somewhat analogous to a cathode ray tube, individual field emission structures are sometimes referred to as vacuum microelectronic triodes. Each triode has the following elements: a cathode (emitter tip), a grid (also referred to as the gate), and an anode (typically, the phosphor-coated element to which emitted electrons are directed).
The phenomenon of field emission was discovered in the 1950's, but it has been only within the last ten years that extensive research and development has been directed toward commercializing the technology. Low-power, high-resolution, high-contrast, monochrome flat panel displays with a diagonal measurement of about 15 centimeters have been manufacturing using field emission cathode array technology. Although useful for such applications as viewfinder displays in video cameras, their small size makes them unsuited for use as computer display screens.
In order for proper display operation, which requires emission of electrons from the cathodes and acceleration of those electrons to a phosphor-coated screen en, an operational voltage differential between the cathode array and the screen of at least 1,000 volts is required. The life of the phosphor coating on the screen increases as the voltage differential increases. Specifically, phosphor coatings on screens degrade as they are bombarded by electrons, with the rate of degradation being proportional to the rate of impact and the total accumulated dose of electrons incident (coulombic aging). As fewer electron impacts are required to achieve a given intensity level at higher voltage differentials, phosphor life can be extended by increasing the operational voltage differential. In order to prevent shorting between the cathode array and the screen, as well as to achieve distortion-free image resolution and uniform brightness over the entire expanse of the screen, highly uniform spacing between the cathode array and the screen is to be maintained.
During tests performed at Micron Display Technology, Inc. in Boise, Id., (presently a division of Micron Technology, Inc.) it was determined that, for a particular evacuated flat-panel field emission display utilizing glass spacer columns to maintain a separation of 250 microns (about 0.010 inches), electrical breakdown occurred within a range of 1,100 to 1,400 volts. All other parameters remaining constant, breakdown voltage will rise as the separation between screen and cathode array is increased. However, maintaining uniform separation between the screen and the cathode array is complicated by the need to evacuate the cavity between the screen and the cathode array to a pressure of less than 10.sup.-6 Torr to enable field emission.
Small area displays (for example, those which have a diagonal measurement of less than 3 centimeters) can be cantilevered from edge to edge, relying on the strength of a glass screen having a thickness of about 1.25 millimeters to maintain separation between the screen and the cathode array. Since the displays are small, there is no significant screen deflection in spite of the atmospheric load. However, as display size is increased the thickness of a cantilevered flat glass screen must be increased exponentially. For example, a large rectangular television screen measuring 45.72 centimeters (18 inches) by 60.96 centimeters (24 inches) and having a diagonal measurement of 76.2 centimeters (30 inches), must support an atmospheric load of at least 28,149 Newtons (6,350 pounds) without significant deflection. A glass screen (or face plate as it is also called) having a thickness of at least 7.5 centimeters (about 3 inches) might well be required for such an application. But that is only half of the problem. The cathode array structure must also withstand a like force without deflection. Although it is conceivable that a lighter screen could be manufactured so that it would have a slight curvature when not under stress, and be completely flat when subjected to a pressure differential, the fact that atmospheric pressure varies with altitude and as atmospheric conditions change makes such a solution impractical.
A more satisfactory solution to cantilevered screens and cantilevered cathode array structures is the use of closely spaced, load-bearing, dielectric (or very slightly conductive, i.e., greater than 10 mega-ohm) spacer structures. Each of the load-bearing structures bears against both the screen and the cathode array plate and thus maintains the two plates at a uniform distance between one another. By using load-bearing spacers, large area evacuated displays might be manufactured with little or no increase in the thickness of the cathode array plate and the screen plate.
Load-bearing spacer structures for field emission array displays generally conform to certain parameters. For instance, the spacer structures should be uniformly non-conductive to prevent catastrophic electrical breakdown between the cathode array and the anode (i.e., the screen). Also, in addition to having sufficient mechanical strength to prevent the flat panel display from imploding under atmospheric pressure, the spacers should exhibit a high degree of dimensional stability under pressure. Furthermore, the spacers should exhibit stability under electron bombardment, as electrons will be generated at each pixel location within the array. In addition, the spacers should be capable of withstanding "bake out" temperatures of about 400.degree. C. that are likely to be used to create the high vacuum between the screen and the cathode array backplate of the display. Further, the material from which the spacers are made should not comprise volatile components which will sublimate or otherwise outgas under low pressure conditions.
For optimum screen resolution, the spacer structures should be nearly perfectly aligned to array topography, and should be of sufficiently small cross-sectional areas so as not to be visible. Cylindrical spacers must typically have diameters no greater than about 50 microns (about 0.002 inch) if they are not to be readily visible. For a single cylindrical lead oxide silicate glass column having a diameter of 25 microns (0.001 inch) and a height of 200 microns (0.008 inch), a buckle load of about 2.67.times.10.sup.-2 Newtons (0.006 pound) has been measured. Buckle loads will of course decrease as height is increased with no corresponding increase in diameter. It is also noted that a cylindrical spacer having a diameter of d will have a buckle load that is only about 18% greater than that of a spacer of square cross-section and a diagonal d, even though the cylindrical spacer has a cross-sectional area about 57% greater than the spacer of square cross-section. If lead oxide silicate glass column spacers having a diameter of 25 microns and a height of 200 microns are to be used in the 76.2 centimeter diagonal display described above, slightly more than one million spacers will be required to support the atmospheric load. To provide an adequate safety margin that will tolerate foreseeable shock loads, that number should probably be doubled in commercially-produced flat panel evacuated displays.
There are a number of drawbacks associated with certain types of spacer structures which have been proposed for use in field emission cathode array-type displays. Spacer structures formed by screen or stencil printing techniques, as well as those formed from glass balls lack a sufficiently high aspect ratio. In other words, spacer structures formed by these techniques must be either so thick that they interfere with the display resolution, or so short that they provide inadequate panel separation for the applied voltage differential. Also, it is generally impractical to form spacer structures by masking and etching deposited dielectric layers in a reactive-ion or plasma environment, as etch steps on the order of 0.250 to 0.625 millimeters would not only greatly hamper manufacturing throughput, but would result in tapered structures (the result of mask degradation during the etch). Likewise, spacer structures formed from lithographically defined photoactive organic compounds are generally unsuitable for application in evacuated flat panel displays as such spacers tend to deform under pressure and to volatilize under both high-temperature and low-pressure conditions. The presence of volatilized substances within the evacuated portion of the display will shorten the life and degrade the performance of the display. Additionally, techniques which adhere stick-shaped spacers to a matrix of adhesive dots deposited at appropriate locations on the cathode array backplate are typically unable to achieve sufficiently accurate alignment to prevent display resolution degradation. Further, any misaligned stick which is adhered to only the periphery of an adhesive dot may later become detached from the dot and fall on top of a group of nearby cathode emitters, thus blocking their emitted electrons. In addition, if an organic epoxy adhesive is utilized for the dots, the epoxy may volatilize over time, leading to the problems heretofore described.
The present invention employs elements of processes disclosed in U.S. Pat. No. 5,486,126 ("the '126 patent", hereby incorporated by reference), as well as elements of processes disclosed in U.S. patent application Ser. No. 08/856,382 (hereby incorporated by reference). The '126 patent teaches the fabrication of an evacuated flat-panel display from specially formed spacer slices. Each spacer slice may be characterized as a matrix which includes permanent, bondable glass fiber strands embedded in a filler material that is selectively etchable with respect to the permanent glass fiber strands. The spacer slices are fabricated by forming a fiber strand bundle having an ordered arrangement of permanent glass fiber strands and filler material strands. The bundle, or a closely packed array of multiple bundles, is sawed into laminar slices and polished to have a final thickness corresponding to a desired spacer height. Multiple spacer slices are positioned on either a display base plate or a display face plate (for a field emission display the face plate is a transparent laminar plate that will be coated with phosphor dots or rectangles; the base plate incorporates the field emitters, as well as the circuitry required to activate the field emitters), to which adhesive (lots have been applied at desired spacer locations thereon. Once the adhesive dots have set up, the filler material within the spacer slices is etched away. Any unbonded permanent spacer columns are also washed away in the etch process. An array of permanent spacer columns remains on the base plate or face plate. The other opposing display plate is then positioned on top of the display plate to which the spacers have been affixed, the cavity between the face plate and the base plate is evacuated, and the edges of the face plate and base plate are sealed so as to hermetically seal the cavity.
Application Ser. No. 08/856,382, like the above-described '126 patent, teaches the fabrication of an evacuated flat-panel display from specially formed spacer slices. However, application Ser. No. 08/856,382, unlike the '126 patent, teaches that spacers are bound to a face plate assembly through anodic bonding processes, and teaches fabrication of spacer slices wherein glass material is utilized for both the spacers and the filler material. The glass filler materials are selectively etchable relative to the glass bonding strands. Such selective etchability can be achieved by having a higher percentage of PbO in the filler glass materials than in the bonding strands. For instance, the bonding strands can have a chemical composition of from about 35% to about 45% PbO, from about 28% to about 35% SiO.sub.2, and a balance of K.sub.2 O, Li.sub.2 O and RbO. In contrast, the filler strands typically have a percentage of PbO that is greater than 50%.
It is preferred that the fiber bonding strands and the filler strands have similar coefficients of expansion, and that the filler strands be selectively etchable relative to the fiber bonding strands. The increased concentration of lead oxide in the filler strands is but one method of accomplishing the above-discussed goals, and other combinations of glass formulations are known that will provide similar coefficients of expansion between glass filler materials and fiber bonding materials, while also enabling selective etchability of the filler materials relative to the bonding materials.
A method of forming a bundle of bonding fibers and filler fibers is to pack the bonding fibers and filler fibers together such that the bonding fibers are surrounded by filler fiber material. An exemplary ratio of bonding fiber strands to filler fiber strands is about 1:3.
Once the fibers are packed together, the bundle can be heated to a sintering temperature (i.e., a temperature at which all constituent fibers fuse together along contact lines or contact surfaces), and then drawn at elevated temperature to uniformly reduce a diameter of all fibers while maintaining a constant relative spacing arrangement between the fibers.
After the bundle is drawn, it can be cut into short intermediate links and redrawn. Ultimately, the drawn bundle has bonding glass fibers within the bundle with a proper diameter (or in other embodiments, rectangular cross-section) for an intended display, with a spacing between the permanent glass fibers corresponding to a spacing between anodic bonding sites of the intended display. The rods can then be packed to form a rectangular block, which is subsequently heated to a sintering temperature in order to fuse the bonding rods and filler rods into a rigid block.
After cooling, the rigid block is sawed into laminar slices. For a 1,500 volt flat-panel field emission display, spacers approximately 380 microns in length (about 0.15 inch) are generally required to safely prevent shorting between the face plate and the base plate. Thus, slices somewhat greater than 400 microns in thickness are cut from the rigid block, and each slice polished smooth on both major surfaces until a final thickness of the block is about 380 microns.
U.S. patent application Ser. No. 08/856,382 further discloses subjecting a laminar silicate glass substrate (soda lime silicate glass can be a preferred material) to a thermal cycle in order to dimensionally stabilize it prior to utilization as a face plate of a field emission display. A disclosed thermal stabilization process encompasses heating the substrate from 20.degree. C. (room temperature) to 540.degree. C. over a period of about three hours. The substrate is maintained at 540.degree. C. for about 0.5 hours. Subsequently, over a period of about one hour, the substrate is cooled to 500.degree. C., and then down to 20.degree. C. over a period of about three hours. A preferred glass substrate has a strain temperature of about 528.degree. C., an anneal temperature of about 548.degree. C., and a transformation temperature (i.e., a temperature above which all silicon tetrahedra that make up the glass have freedom of rotational movement) of about 551.degree. C.
After the silicate glass substrate is subjected to the thermal cycle, it is subjected to further processing to provide anodic bonding sites across a surface of the material, and to anodically bond the glass fiber bonding strands to the anodic bonding sites. Subsequently, the glass filler materials are etched away utilizing, for example, an acid bath having a temperature of from 20.degree. C. to 40.degree. C., and comprising from about 2% to about 10% hydrogen chloride in deionized water. The duration of the wet etch can vary from about 0.5 hours to about four hours, with the duration depending, at least in part, on an amount of agitation and a thickness of filler glass that is to be etched away. After removal of the filler material, any bonding strands that have not adhered to a desired bonding site on the silicate glass substrate are removed. The remaining silicate glass face plate with bonding strands provided only at desired bonding sites is then incorporated into a field effect display device. The bonding strands function as spacer structures within the device.
It would be desirable to develop alternative methods of incorporating load-bearing structures into field emission cathode array-type displays. The spacer structures should preferably be aligned in desired locations between a face plate and back plate.